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Semaphores: Provide macros for sem_setprotobol() and sem_getprotocol() if priority inheritance is not enabled. More SEM_PRIO_* definitions to include/nuttx/semaphore.h

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This commit is contained in:
Gregory Nutt 2016-11-02 18:21:46 -06:00
parent d5b4d848d3
commit 97bf82ee05
5 changed files with 120 additions and 32 deletions
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51
TODO
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@ -9,7 +9,7 @@ issues related to each board port.
nuttx/:
(13) Task/Scheduler (sched/)
(14) Task/Scheduler (sched/)
(1) Memory Management (mm/)
(1) Power Management (drivers/pm)
(3) Signals (sched/signal, arch/)
@ -216,6 +216,55 @@ o Task/Scheduler (sched/)
Status: Open
Priority: Medium-ish
Title: ISSUES WITH PRIORITY INHERITANCE WHEN SEMAPHORE USED AS IPC
Description: The semaphores have multiple uses. The typical usage is where
the semaphore is used as lock on one or more resources. In
this typical case, priority inheritance works perfectly: The
holder of a semaphore count must be remembered so that its
priority can be boosted if a higher priority task requires a
count from the semaphore. It remains the holder until is
calls sem_post() to release the count on the semaphore.
But a different usage model for semaphores is for signalling
events. In this case, the semaphore count is initialized to
zero and the receiving task calls sem_wait() to wait for the
next event of interest. When an event of interest is
detected by another task (or even an interrupt handler),
sem_post() is called which increments the count to 1 and
wakes up the receiving task.
For example, in the following TASK A waits for events and
TASK B (or perhaps an interrupt handler) signals task A of
the occurence of the events by posting the semaphore:
TASK A TASK B
sem_init(sem, 0, 0);
sem_wait(sem);
sem_post(sem);
Awakens as holder
These two usage models are really very different and priority
inheritance simply does not apply when the semaphore is used for
signalling rather than locking. In this signalling case
priority inheritance can interfere with the operation of the
semaphore. The problem is that when TASK A is awakened it is
a holder of the semaphore. Normally, a task is removed from
the holder list when it finally release the
However, TASK A never calls sem_post(sem) so it becomes
*permanently* a holder of the semaphore and may have its
priority boosted when any other task tries to acquire the
semaphore.
The fix is to call sem_setprotocol(SEM_PRIO_NONE) immediately
after the sem_init() call so that there will be no priority
inheritance operations on this semaphore used for signalling.
Status: Open
Priority: High. If you have priority inheritance enabled and you use
semaphores for signalling events, then you *must* call
sem_setprotocol(SEM_PRIO_NONE) immediately after initializing
the semaphore.
Title: SCALABILITY
Description: Task control information is retained in simple lists. This
is completely appropriate for small embedded systems where

33
include/nuttx/semaphore.h
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@ -51,6 +51,12 @@
* Pre-processor Definitions
****************************************************************************/
/* Values for protocol attribute */
#define SEM_PRIO_NONE 0
#define SEM_PRIO_INHERIT 1
#define SEM_PRIO_PROTECT 2
/****************************************************************************
* Public Type Definitions
****************************************************************************/
@ -67,9 +73,7 @@ struct nsem_inode_s
sem_t ns_sem; /* The contained semaphore */
/* Inode payload unique to named semaphores. ns_inode must appear first
* in this structure in order to support casting between type sem_t and
* types of struct nsem_inode_s. */
/* Inode payload unique to named semaphores. */
FAR struct inode *ns_inode; /* Containing inode */
};
@ -158,6 +162,8 @@ int sem_reset(FAR sem_t *sem, int16_t count);
#ifdef CONFIG_PRIORITY_INHERITANCE
int sem_getprotocol(FAR sem_t *sem, FAR int *protocol);
#else
# define sem_getprotocol(s,p) do { *(p) == SEM_PRIO_NONE); } while (0)
#endif
/****************************************************************************
@ -166,6 +172,25 @@ int sem_getprotocol(FAR sem_t *sem, FAR int *protocol);
* Description:
* Set semaphore protocol attribute.
*
* One particularly important use of this function is when a semaphore
* is used for inter-task communication like:
*
* TASK A TASK B
* sem_init(sem, 0, 0);
* sem_wait(sem);
* sem_post(sem);
* Awakens as holder
*
* In this case priority inheritance can interfere with the operation of
* the semaphore. The problem is that when TASK A is restarted it is a
* holder of the semaphore. However, it never calls sem_post(sem) so it
* becomes *permanently* a holder of the semaphore and may have its
* priority boosted when any other task tries to acquire the semaphore.
*
* The fix is to call sem_setprotocol(SEM_PRIO_NONE) immediately after
* the sem_init() call so that there will be no priority inheritance
* operations on this semaphore.
*
* Parameters:
* sem - A pointer to the semaphore whose attributes are to be
* modified
@ -179,6 +204,8 @@ int sem_getprotocol(FAR sem_t *sem, FAR int *protocol);
#ifdef CONFIG_PRIORITY_INHERITANCE
int sem_setprotocol(FAR sem_t *sem, int protocol);
#else
# define sem_setprotocol(s,p) DEBUGASSERT((p) == SEM_PRIO_NONE);
#endif
#undef EXTERN

27
include/pthread.h
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@ -40,18 +40,19 @@
* Included Files
********************************************************************************/
#include <nuttx/config.h> /* Default settings */
#include <nuttx/compiler.h> /* Compiler settings, noreturn_function */
#include <nuttx/config.h> /* Default settings */
#include <nuttx/compiler.h> /* Compiler settings, noreturn_function */
#include <sys/types.h> /* Needed for general types */
#include <sys/prctl.h> /* Needed by pthread_[set|get]name_np */
#include <sys/types.h> /* Needed for general types */
#include <sys/prctl.h> /* Needed by pthread_[set|get]name_np */
#include <stdint.h> /* C99 fixed width integer types */
#include <stdbool.h> /* C99 boolean types */
#include <unistd.h> /* For getpid */
#include <semaphore.h> /* Needed for sem_t */
#include <signal.h> /* Needed for sigset_t, includes this file */
#include <time.h> /* Needed for struct timespec */
#include <stdint.h> /* C99 fixed width integer types */
#include <stdbool.h> /* C99 boolean types */
#include <unistd.h> /* For getpid */
#include <signal.h> /* Needed for sigset_t, includes this file */
#include <time.h> /* Needed for struct timespec */
#include <nuttx/semaphore.h> /* For sem_t and SEM_PRIO_* defines */
/********************************************************************************
* Pre-processor Definitions
@ -112,9 +113,7 @@
#define PTHREAD_INHERIT_SCHED 0
#define PTHREAD_EXPLICIT_SCHED 1
#define PTHREAD_PRIO_NONE 0
#define PTHREAD_PRIO_INHERIT 1
#define PTHREAD_PRIO_PROTECT 2
/* Default priority */
#define PTHREAD_DEFAULT_PRIORITY 100
@ -135,7 +134,7 @@
#define PTHREAD_BARRIER_SERIAL_THREAD 0x1000
/* Values for protocol attribute */
/* Values for protocol mutex attribute */
#define PTHREAD_PRIO_NONE SEM_PRIO_NONE
#define PTHREAD_PRIO_INHERIT SEM_PRIO_INHERIT

22
include/semaphore.h
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@ -45,24 +45,10 @@
#include <stdint.h>
#include <limits.h>
#ifdef __cplusplus
#define EXTERN extern "C"
extern "C"
{
#else
#define EXTERN extern
#endif
/****************************************************************************
* Pre-processor Definitions
****************************************************************************/
/* Values for protocol attribute */
#define SEM_PRIO_NONE 0
#define SEM_PRIO_INHERIT 1
#define SEM_PRIO_PROTECT 2
/* Bit definitions for the struct sem_s flags field */
#define PRIOINHERIT_FLAGS_DISABLE (1 << 0) /* Bit 0: Priority inheritance
@ -130,6 +116,14 @@ typedef struct sem_s sem_t;
* Public Data
****************************************************************************/
#ifdef __cplusplus
#define EXTERN extern "C"
extern "C"
{
#else
#define EXTERN extern
#endif
/****************************************************************************
* Public Function Prototypes
****************************************************************************/

19
sched/semaphore/sem_setprotocol.c
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@ -58,6 +58,25 @@
* Description:
* Set semaphore protocol attribute.
*
* One particularly important use of this furnction is when a semaphore
* is used for inter-task communication like:
*
* TASK A TASK B
* sem_init(sem, 0, 0);
* sem_wait(sem);
* sem_post(sem);
* Awakens as holder
*
* In this case priority inheritance can interfere with the operation of
* the semaphore. The problem is that when TASK A is restarted it is a
* holder of the semaphore. However, it never calls sem_post(sem) so it
* becomes *permanently* a holder of the semaphore and may have its
* priority boosted when any other task tries to acquire the semaphore.
*
* The fix is to call sem_setprotocol(SEM_PRIO_NONE) immediately after
* the sem_init() call so that there will be no priority inheritance
* operations on this semaphore.
*
* Parameters:
* sem - A pointer to the semaphore whose attributes are to be
* modified